The present invention relates to scrambling/descrambling techniques for data transmission. The present invention is particularly useful for communications systems using pseudo-random bit patterns in which the same pattern appears recyclically at relatively long intervals.
According to a known scrambling technique, data bits are organized into successive frames of a multiframe format and the data bits of each frame are scrambled at a transmit end of a communications system with a pseudo-random sequence which varies with each frame and repeats the same pattern at intervals of multiframes. At the receive end of the system, the data bits of each frame are descrambled using the same pseudo-random bit pattern. However, if frame synchronization is established at the receive end at an intermediate point of a given multiframe, valid descrambling cannot take place until the beginning of the next multiframe. As a result, a substantial amount of data must be discarded even if the frame synchronization has been established. This problem is particularly acute when data transmission speed is relatively low, typically at 9.6 kbps, since the time taken to obtain the valid timing for descrambling is substantial.